Answer:

Explanation:
The information that we kow about the shape of the aquarium:
length:

width:

when the reef is submerged, the water level rises 3 feet. The volume that those 3 ft represent in the aquarium will be the volume of the reef.
Thus, we calculate the volume of water "addition" with the same length and width as the aquarium but with a deep of 3 feet (thi is because the reef caused the water to rise 3 feet), and this is the volume of the reef that was added to the aquarium:

The volume of the stone is
